// 只返回协议类型    
public function is_https() {
        if ( !empty($_SERVER['HTTPS']) && strtolower($_SERVER['HTTPS']) !== 'off') {
            return 'https';
        } elseif ( isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https' ) {
            return 'https';
        } elseif ( !empty($_SERVER['HTTP_FRONT_END_HTTPS']) && strtolower($_SERVER['HTTP_FRONT_END_HTTPS']) !== 'off') {
            return 'https';
        }
        return 'http';
    }
// 返回协议类型+域名
function is_https()
{
    if (!empty($_SERVER['HTTPS']) && strtolower($_SERVER['HTTPS']) !== 'off') {
        return 'https';
    } elseif (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https') {
        return 'https';
    } elseif (!empty($_SERVER['HTTP_FRONT_END_HTTPS']) && strtolower($_SERVER['HTTP_FRONT_END_HTTPS']) !== 'off') {
        return 'https' . '://' . $_SERVER['HTTP_HOST'];
    }
    return 'http' . '://' . $_SERVER['HTTP_HOST'];
}